The winter problem of hydropower

The original article can be read as “Schlumpfs graphic 117” in the online Nebelspalter of 1 July 2024.

Switzerland’s 700 or so hydropower plants form a huge and well-coordinated energy system that generates more than half of our electricity. It is important to take a closer look at this system in order to correct misconceptions about its potential. Because the “fuel” of these power plants, water, is naturally not always available in the same quantities, we are dealing with volatile electricity production, which is also characterized by a clear winter weakness. This is because significantly less water flows down the streams and rivers in winter than in summer.

What is important:

– Without reservoirs, only 26 percent of hydroelectricity could be generated in the winter months. With the reservoirs, the figure is 43 percent.
– Nevertheless, hydropower has a winter problem – precisely when the demand for electricity is at its highest.
– Solar panels are the most unfavorable supplement to hydropower because they also produce much less electricity in winter.

All figures in this article come from statistics from the Swiss Federal Office of Energy (SFOE, see here). I have used these sources to calculate the average for the last ten years (2014-2023) for all charts.

The electricity yield from hydropower fluctuates greatly

Electricity generation from hydropower, known as hydraulic power generation, is the main pillar of our electricity system: on average over the last ten years, hydropower plants generated 38 terawatt hours (TWh) of electricity per year, covering 62 percent of the country’s consumption. However, there were strong fluctuations: While the hydropower plants generated as much as 40.8 TWh of electricity in 2023 (68 percent of the country’s consumption), the figure for 2022 was only 33.5 TWh (55 percent of the country’s consumption). In the long-term analysis since 1980, the minimum electricity yield of hydropower is just under 30 TWh and its peak value is 42 TWh: Such huge differences in production create a great deal of uncertainty for our electricity system.

Hydroelectricity is generated in Switzerland by two different types of power plant: Run-of-river power plants and storage power plants. Run-of-river power plants are generally river power plants whose production possibilities depend directly on the water conditions and are therefore not controllable. In contrast, storage power plants use the storage of water in a reservoir for electricity production that can be controlled over time. This means that a large part of the electricity potential of natural water inflows from rain and glacier meltwater, which are much more productive in summer, can be shifted to winter.

Reservoirs increase the proportion of water flow in winter to 43 percent

Every year, specialists from the Swiss Federal Office of Energy (SFOE) calculate the extent to which this storage improves hydraulic power generation in winter compared to summer. The next graph shows the effect of this seasonal storage on a ten-year average:

The two light blue bars in the graph show the electricity shares for the winter and summer half-years as if Switzerland had no storage power plants – i.e. as if electricity were generated directly from natural inflows. The two dark blue bars represent the shares of real production using the storage power plants. The result is impressive: the share of hydropower in winter can be increased from 26 percent to 43 percent compared to summer thanks to the reservoirs.

Winter weakness despite reservoirs

However, this also means that electricity generation from hydropower using all reservoirs is still weak in winter: Three units in winter are offset by four units in summer. But what does this hydraulic winter weakness look like in detail with regard to the shares of the run-of-river and storage plants? The next graph shows the monthly generation of these two types of power plant on a ten-year average:

The blue bars below show the monthly electricity generation from run-of-river (dark blue) and storage power plants (light blue) in gigawatt hours (GWh). It can be seen how the run-of-river plants supply much less electricity in the winter months than in the summer. In contrast, the storage power plants compensate as much as they can in winter: From November to January, they generate high values of around 2000 GWh per month, after which their production falls because the reservoirs empty and there are still few natural inflows.

Only 36 percent comes from run-of-river power plants in winter

In summary, the run-of-river power plants only contribute 36 percent to electricity generation from hydropower in the winter half-year, while in the summer half-year they are on a par with the storage plants. The total of these two types of production is shown in the graph with the dark blue curve: Switzerland’s hydraulic power generation is at its lowest from February to April at 2500 GWh, then rises to peak values of over 4000 GWh per month until the summer, before falling back to 3000 GWh by the start of winter.

The decisive factor now is how this hydroelectricity generation curve relates to Switzerland’s national electricity consumption. This is shown in the next graph:

Electricity consumption and hydropower generation go in opposite directions

In this graph, the already known blue curve of hydraulic power generation is compared with the monthly values of our national consumption (red curve) on a ten-year average. As can be seen, the two curves behave in opposite directions in a seasonal comparison. This means that the monthly differences – i.e. the winter electricity gap in hydropower – can now be quantified: While practically all consumption can be covered by hydropower in the summer months of June to August, an increasingly large gap opens up by the beginning of winter, which reaches 3000 GWh from December to March and then disappears again in the rest of spring.

Solar is the worst supplement to water

Two points as a conclusion:

1. Because our electricity system has to use other production sources in addition to hydropower, the question arises as to which energy source is best suited to supplement hydropower in terms of security of supply. Our ancestors chose nuclear energy for this purpose: This is the ideal supplement, with its reliable continuous power production in winter. On the other hand, fluttering electricity from solar plants is the worst possible option, as it massively increases the winter electricity shortfall of hydroelectricity. This is because solar panels also generate the least electricity in winter.
2. Because hydropower only achieves a share of 43 percent in winter if all reservoirs are used, all claims that surplus electricity from solar systems could be transferred from summer to winter through our reservoirs are smoke and mirrors: anything that could be done in this respect would be at the expense of storing hydropower itself. Only the raising or construction of new dams or the installation of new pumps or turbines could increase the electricity potential of hydropower.